Many known optical mark or character-reading devices rely on simple video circuits for sensing contrasting marks or characters appearing on documents for producing electrical signals which are interpreted to yield information about the mark or character sensed. Video circuits of this type commonly require high-contrast marks or characters against clean, white background to assure proper operation and correct sensing of the mark or character. Certain threshold levels are usually selected in such video circuits in order to optimize the accuracy of sensed information for the conditions of darkness or density of the marks or characters to be sensed, the background color of the document, the level of background illumination used, the speed of character scanning, and the like. However, such simple video circuits commonly suffer degradation in sensing accuracy for a given setting of threshold levels as a result of changes in such parameters as the density of marks or characters being sensed, the background color, the level of illumination, the scanning speed, the type or font of characters being sensed, and the like.